Universal Device Pollers (UDPs) are tied to the polling engine on which they are created. This means if a UDP is created on the main polling engine, it will only collect data for devices polled by that engine. To collect data from devices polled by an additional polling engine, you must create or assign the UDP on that engine as well.
Exact Extract from Official Guide:
"Universal Device Pollers are tied to the polling engine on which they are created. To monitor devices assigned to an Additional Polling Engine, the UDP must be created or assigned on that engine."
- (Network Performance Monitor Administrator Guide, "Universal Device Pollers" section) References:
Network Performance Monitor Administrator Guide, Universal Device Pollers section